Nuclear fusion takes place when nuclei of atoms collide at high energies to form a nucleus which is more complex but whose rest mass is less than the sum of the rest masses of the original nuclei. To achieve the energies necessary to enable fusion processes to take place, the gas serving as fuel must be heated to extremely high temperatures (millions of degrees Farenheit) whereby the gas becomes completely ionized as a plasma, i.e., all the atoms split up into freely moving electrons and ions. The plasma which is maintained within a vacuum chamber is compressed into a relatively small space by the action of a sufficiently strong magnetic field while being maintained out of contact with the inner wall of the chamber. Because charged particles tend to gyrate along magnetic field lines, the resulting plasma can be confined in a suitably shaped magnetic containment structure, such as a toroid. If a plasma is to be confined in the shape of a torus, however, the magnetic bottle confining it cannot consist only of a toroidal magnetic field. The particles themselves would set up an electric field normal to the magnetic field and under the action of this field and the existing toroidal magnetic field would drift out of the plasma.
As a solution, in the tokamak approach, an electric current is induced along the plasma column flowing the long way around the torus. The associated poloidal magnetic field gives rise to helical field lines which form a set of nested magnetic surfaces. Under these conditions the particles moving along the helical field lines will drift an equal distance away from and towards the center of the plasma during each pass around the torus thus maintaining, on the average, a constant radial position. In practice, the duration of sustainment of the plasma is very short, that is, much less than one second. The hot ions near the center of the plasma, under the influence of collisions, tend to flow out of the plasma. Energy flows out of the plasma by heat conduction. Impurities, which exist in the plasma from, among other sources, the surface of the inner wall of the containing device structure, tend to radiate away (quench) energy and thereby reduce the plasma temperature.
In order to provide a long pulse sustainment capability, e.g., a pulse length of at least 30 seconds with plasma currents in the order of megamperes, it is necessary to provide impurity control. The cross section of the plasma is shaped by sets of poloidal coils to maintain a closed form. Impurities within the plasma tend to flow outwardly toward the surface of the heat shield and enter a "scrape off" region wherein the outer surface of the plasma and impurities are diverted to a lower collector region for impingement on a target. Energy is removed from the scrape-off plasma and its entrained impurities by cooling on the surface of the target. The plasma particles and impurities are then removed by conventional vacuum gettering.
Because the thermal power of the scrape off plasma entering the scrape off region is extremely high, a heat transfer device capable of withstanding the large power flux of the scrape off plasma is required for the divertor target. The target must have a very high burnout heat flux characteristic and must also be actively cooled to remove sufficient energy from the scrape off plasma and impurities to enable removal by conventional gettering techniques.
A general object of the present invention, therefore, is to provide a new and improved divertor target for a plasma magnetic containment device.
A more particular object of the invention is to provide an actively cooled divertor target having a very high burnout characteristic to enable cooling of the scrape off flux and its entrained impurities in a manner whereby none of the scrape off flux impinges directly on other parts of the lower collector region.
Another object is to provide an actively cooled divertor target that is geometrically arranged such that impinging heating flux is maintained at a minimum.
Still another object is to provide an actively cooled divertor target having a minimum number of tubes in order to reduce maintenance problems.